1. Fields of the Invention
The present invention relates to cellular communication, and more specifically to an output power control system for a cellular base station.
2. Relevant Background
A cellular communication system divides a geographic region into several smaller-sized service areas called xe2x80x9ccellsxe2x80x9d. Typically, each cell includes a base station which communicates with a number of mobile stations within the cell. A conventional base station generally includes an external amplifier which boosts an input signal to an appropriate power level for transmission. The boosted signal from the external amplifier is then passed to an antenna via a cable and transmitted to the mobile stations.
An important aspect of cellular communication is the amount of radio interference between closely located base stations. Radio interference is typically caused by two or more broadcasting stations simultaneously transmitting radio signals which overlap in both frequency and geography. For example, if two television stations broadcast the same channel frequency in the same city, each station signal will interfere with the other, resulting in information loss due to the clashing signals. Radio interference between base stations is therefore undesirable during wireless communication.
Minimizing interference between radio stations operating at the same carrier frequency range requires controlling the geographic service area covered by each station such that there is minimal overlap between service areas. A station""s service area is generally proportional to the power of its transmitted signal, with the territorial coverage increasing as transmission power is increased. By decreasing the transmission power, less area is covered by the transmitting station.
As previously mentioned, a cellular system utilizes cells to achieve radio communication. Ideally, each cell""s effective service area boundary just touches a neighboring cell""s boundary, thereby forming a grid pattern over the entire region of the cellular network. In order to minimize interference between adjoining cells, each base station""s transmission power must be decreased to cover an area no greater than the cell""s allocated service area. Decreasing the power of transmitted radio signals too much, however, can be problematic. Typically, if the power level of transmitted radio signals is too low, poor signal to noise ratios at the receiving end result. This can often lead to insufficient cell coverage or a received signal which contains static and/or errors. Therefore, the ability to precisely control base station transmission power is vital to a cellular communication system.
In addition to minimizing interference between base stations, transmission power levels are also adjusted as cellular traffic in a cell changes. Typically, each base station can service up to a certain maximum number of mobile stations at any given time. Once a base station reaches its maximum capacity, the introduction of additional mobile stations requiring service by the base station often leads to blocked calls, dropped calls, and a general feeling of hostility towards the cellular service provider by its customers.
To prevent such undesirable consequences from occurring, base station transmission power is often adjusted to accommodate high cellular traffic areas. Such adjustments may help shift a base station""s service area from a region of high cellular traffic to a region of lower cellular traffic. Furthermore, power adjustments to existing base stations are often necessary when new base stations are constructed to meet the changing traffic demands of a cellular system.
In addition to meeting the changing traffic demands of cellular systems, precise control of base station transmission power levels is also important in meeting the changing demands of government agencies. Cellular radio communication is typically government regulated for various frequency bands and geographic areas. Government regulations often include a maximum allowable transmission power level which base stations must not exceed. It is therefore necessary to accurately check and control power levels of base station transmitted signals in order to comply with government regulations.
Although a base station""s transmission power level is an important specification in cellular communication systems, accurately adjusting transmission power is often difficult to perform. Component non-linearity typically requires trial and error procedures when tuning a base station to an appropriate power transmission level. In addition, parameter variations of base station components during manufacturing typically prevent identical base station adjustments from yielding the same power transmission levels from one base station to another.
Other difficulties may further develop once base stations are installed at their intended site location. After a base station is moved to its permanent location, field equipment is generally necessary to adjust base station transmission power levels. Typically, field equipment is less accurate and more prone to calibration errors due to travel wear and tear than stationary test equipment used during production. In addition, it is often difficult and expensive to carry field equipment to remote site locations, such as a mountain peak. Moreover, adjusting transmission power levels of active base sites may also require interruption of service during testing. It is therefore important to accurately perform transmission power level adjustments as quickly as possible in some cases.
What is therefore needed is a base station design which allows transmission power levels to be adjusted quickly and accurately. The design should take into account component non-linearities and parameter variations. The design should also allow for power level adjustments without requiring the transport of field test equipment.
Briefly stated, the present invention involves a transmitter circuit for accurately amplifying a data signal to a desired power level. The transmitter circuit includes a variable attenuator coupled to the data signal such that the data signal is attenuated by an attenuation factor proportional to a reference signal. The transmitter circuit further includes an external amplifier coupled to the variable attenuator for amplifying the data signal to an external amplifier power level. The external amplifier includes a forward power detector and a calibration table. The forward power detector provides a forward power signal to the calibration table. The calibration table associates the forward power signal to the external amplifier power level. A controller configured to receive the external amplifier power level from the calibration table modifies the reference signal in response to a difference between the amplifier power level and the desired power level.
Another aspect of the invention is a method for transmitting a data signal at a desired power level for a radio transmitter. The method includes storing a group of calibrated power level values in a power lookup table, measuring an amplified voltage level of the data signal, providing a calibrated power level selected from the group of calibrated power level values based on the amplified voltage level, using the difference between the calibrated power level and the desired power level to generate an attenuation control value, and attenuating the data amplitude according to the attenuation control value.